1. Field of the Invention
The invention relates to programming read-only memory (ROM) cells in MOS or CMOS technology.
2. Discussion of the Related Art
A programmed read-only memory cell and a non-programmed read-only memory cell show electrically different behavior when they are subjected to the same read signals, i.e., one of them conducts very little current or even no current at all, while the other conducts high current, at least ten to one hundred times higher than the former. Hereinafter, it shall be assumed, by convention, that the programmed cell is the one that conducts no current.
The manufacturers of read-only memories are particularly interested in methods that enable programming in the upper layers of the circuit, i.e., that come into play at an advanced stage of manufacture of the integrated circuit, even for example in the final layers.
In this field, there is in particular a known way of carrying out a drain contradoping operation for a transistor in a drain region adjacent to the conduction channel so as to have a reversed bias junction at the drain. This reversed bias junction, which is located upline with respect to the transistor, prevents the passage of the current in read mode. This programming method and the associated manufacturing method are described in the document EP 92830552.3 which is herein incorporated by reference.
This technique requires the making of a basic transistor (non-programmed) with a weakly doped drain and the carrying out a deep (not surface) contradoping implantation to obtain a reversal of the type of doping throughout the depth of the region of the drain considered. With a more highly doped drain, there would remain a channel with the initial type of doping beneath this region and the desired effect would not be obtained. However, this weak doping of the drain and hence the low mobility of the carriers in the drain gives rise to high access resistance in the transistor and hence low current in reading mode even if the source is more highly doped in order to improve the conduction.
Furthermore, depending on the junction gradient obtained, which depends on the energy and the profile of implantation of the contradoping, it is possible to have a junction with a high breakdown voltage but also with substantial leakage currents, especially under heat. Or else, on the contrary, there may be a junction with leakage currents almost equal to zero, but then with a low breakdown voltage (abrupt junction). Now, firstly, the leakage currents are very inconvenient, and secondly if the breakdown voltage is very low, the junction will conduct current in reverse in read mode and the transistor will be read as being non-programmed. This is very inconvenient and calls for very painstaking operations to adjust the energy and doping levels in order to minimize the leakage currents while at the same time having sufficiently high breakdown voltage. The reproducibility of this programming method and hence its reliability are thereby affected.
An object of the invention is to provide a method of programming that does not have these different drawbacks, and an associated memory.
Another object of the invention is to provide a method of programming that can be carried out at an advanced stage of manufacture and is reliable and reproducible, and an associated memory.